(a) Field of the Invention
The present invention relates to a fibrous electrically-conductive filler and a process for producing the filler and more specifically to a fibrous electrically-conductive filler which is admixed with a substance such as papers, plastics, rubbers, resins or paints to impart electrical conductivity thereto and which has a low volume specific resistivity and an excellent electrical conductivity-imparting effect per unit weight thereof, as well as a process for producing the same.
(b) Description of the Prior Art
There have been known various fine substances which can be added to, for instance, plastics and paints to impart electrical conductivity thereto, for instance, such as tin oxide powder doped with antimony, zinc oxide powder doped with aluminum, or powder, for instance, titanium oxide or aluminum oxide covered with tin oxide as well as fibrous substances, for instance, glass fibers, fibers of alkali metal titanates and titanium oxide fibers covered with tin oxide.
As has been well-known, electrically-conductive fine substances must come in contact with one another in a substance to which they are added for imparting excellent electrical conductivity to the substances such as plastics and paints. For this reason, it is necessary to add a great deal of powder to these materials if electrically-conductive spherical powder is employed. Thus, products obtained by admixing expensive electrically-conductive powder to materials are limited in their applications because of the production cost thereof.
To eliminate this problem, there have been proposed the aforementioned fibrous substances having an aspect ratio substantially greater than that for the spherical powder and a shape which makes it possible to increase the probability of contact between the conductibity-imparting substances even in a relatively small added amount. For instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") Nos. Sho 59-102820 and Sho 62-59528 disclose electrically-conductive alkali metal titanate fibers comprising alkali metal titanate fibers represented by the general formula: M.sub.2 O.multidot.nTiO.sub.2 .multidot.mH.sub.2 O coated with tin oxide substance as well as a process for producing the same. In addition, J.P. KOKAI No. Sho 62-122005 discloses a process for producing a white fibrous electrically-conductive filler, the process comprising liquating out alkaline compoments which adversely affect electrical conductivity through a preliminary treatment of alkali metal titanate fibers with an acid and then coating the fibers with a tin oxide substance.
As compared with electrically-conductive fibers obtained by coating glass fibers with a electrically-conductive substance, if these conventional fibrous electrically-conductive alkali metal titanate fillers are admixed with a substance such as a plastic, a rubber or a paint, only a small amount of the fibrous substance is peeled off from the electrically-conductive material the electrical conductivity-imparting effect thereof is only slightly decreased. However, the alkali metal titanate fibers per se have a relatively low resistance to acids and this, in turn, lead to various problems. For instance, the alkali components thereof liquate out due to the action of an acid in coating the fibers with a electrically-conductive substance, the fibers are easily broken, the diameter thereof is reduced, they become porous and correspondingly the surface area thereof is undesirably increased. For this reason, it is required to use a large amount of a electrically-conductive substance for forming a electrically-conductive layer having the same thickness as that for the conventional fillers.
Moreover, the electrical conductivity of products obtained by incorporating a electrically-conductive filler is in general dependent upon the amount of the electrically-conductive filler (volume %) and thus, the smaller the specific gravity of a electrically-conductive filler the smaller the amount of the filler to be incorporated. In this respect, however, the foregoing alkali metal titanate fibers are not always sufficient.